A fine particulate titanium dioxide has been heretofore used in various applications such as a UV-shielding material, an additive to silicone rubber, a dielectric raw material and a cosmetic material. In recent years, application to photocatalysts, solar cells and the like is attracting much attention.
As for the crystal form of titanium dioxide, three types of rutile, anatase and brookite, are present and among these, anatase or brookite titanium dioxide more excellent in the photoelectrochemical activity than the rutile is used in the fields of photocatalyst and solar cell.
The photocatalytic activity of titanium dioxide is utilized, for the decomposition of organic materials, as antimicrobial tiles, self-cleaning building materials and deodorant fibers, and the mechanism thereof is understood to be as follows. The titanium dioxide absorbs ultraviolet light and generates an electron and a hole in the inside thereof. The hole reacts with the adsorbed water of titanium dioxide to produce a hydroxy radical and, by the effect of this radical, the organic material adsorbed to the surface of the titanium dioxide particles is decomposed into carbonic acid gas or water (Akira Fujishima, Kazuhito Hashimoto and Toshiya Watanabe, Hikari Clean Kakumei (Light Clean Revolution), CMS (1997)).
That is, the conditions required of the titanium dioxide having strong photocatalytic activity are to readily generate a hole and to allow the hole to easily reach the titanium dioxide surface. Examples of the titanium dioxide having high photocatalytic activity include those of anatase type, those having a small number of crystal defects, and those of giving a small particle having a large specific surface area (Kazuhito Hashimoto and Akira Fujishima (compilers), Sannka Titan Hikari Shokubai no Subete (All About Titanium Oxide Photocatalyst), CMC (1998)).
In practice, titanium dioxide is fixed to the surface of a substrate with a binder and when light is irradiated onto that layer, a catalytic activity is realized. Transparency of a photo-catalytic layer is demanded for the aesthetic reasons. Accordingly, when titanium dioxide is supported on a substrate, the amount of titanium dioxide and dispersibility of the powder are very important.
As for the application to solar cells, a dye-sensitized solar cell comprising a combination of a titanium dioxide and a ruthenium-base dye was reported in 1991 by Graetzel et al. of EPFL-Lausanne and, since this discovery, studies are being made thereon (M. Graezel, Nature, 353, 737 (1991)).
In the dye-sensitized solar cell, the titanium dioxide plays the role of a support for the dye as well as of an n-type semiconductor and is used as a dye electrode bound to an electrically conducting glass electrode. The dye-sensitized solar cell has a structure where an electrolytic layer is sandwiched by a dye electrode and a counter electrode, where the dye absorbs light and thereby generates an electron and a hole. The electron generated is transferred to the electrically conducting glass electrode through the titanium dioxide layer and taken outside. On the other hand, the generated hole is transferred to the counter electrode through the electrolytic layer and combines with an electron supplied through the electrically conducting glass electrode. One of the factors for improving the characteristic feature of a dye-sensitized solar cell is that the titanium dioxide and the dye are easily combined. As for the crystal form of titanium dioxide which can be easily combined with the dye, for example, JP-A-10-255863 (the term “JP-A” as used herein means an “Japanese Unexamined Patent Publication (Kokai)”) describes use of an anatase type, and JP-A-2000-340269 states that a brookite type is suitable for dye-sensitized solar cells.
To bring out the function of titanium dioxide, good dispersibility is important. For example, when the titanium dioxide is used as a photocatalyst, if the dispersibility is bad, the covering property is intensified and the usable application is restricted. A titanium dioxide having bad dispersibility hardly transmits light and, therefore, also in the field of solar cells, the amount of titanium dioxide capable of contributing to the light absorption is limited and the photoelectric conversion efficiency decreases. In general, it is considered that light scattering (covering power) becomes maximum when the particle diameter is about a half of the visible light wavelength, and as the particle size becomes smaller, the light scattering is weakened (Manabu Kiyono, Sannka-Titan (Titanium Oxide), p. 129, Gihodo-Shuppan (1991)).
The primary particle diameter of the titanium dioxide used in the above-described field is from several nm to tens of nm in many cases and, therefore, as long as the dispersibility is good, the effect on the light scattering is small. If the titanium dioxide has poor dispersibility and gives an aggregated particle having a large diameter, light scattering is intensified. Therefore, the particle having good dispersibility can be said to be a particle which is free from aggregation and can be stably present in a state close to a primary particle in a solvent.
The titanium dioxide is an indispensable material as a high-performance dielectric raw material. The dielectric material, for example, BaTiO3 is obtained by the following reaction under heating:BaCO3+TiO2→BaTiO3+CO2 
In order to enhance the dielectric property of BaTiO3, the BaTiO3 particle must be pulverized. The reaction above is a solid phase reaction and it is said that BaCO3 is first decomposed at a high temperature to produce BaO, and the BaO is diffused and solid-dissolved in the TiO2 particle and becomes BaTiO3. Accordingly, the size of the BaTiO3 particle is governed by the size of the TiO2 particle. The chlorine contained in the TiO2 particle is present by adsorbing on an extreme surface layer of the particle and reacts with BaO produced during heating to produce BaCl2. This BaCl2 is melted and acts as a flux to bring about aggregation of TiO2 particles or BaTiO3 particles. Also, the melted flux is readily localized and many aggregations occur in the localized portion, as a result, the quality differs from other portions. In addition, when the particles are aggregated, the BaTiO3 particle crystal grows into an abnormal particle and thus decreases the dielectric property of BaTiO3. during the synthesis of a high-performance dielectric material, the ratio of BaO and TiO2 must be strictly controlled to be 1:1, but the presence of chlorine causes deviation from the compositional ratio.
Furthermore, fluctuation of the adsorbed water on the particle surface gives rise to a problem greater than the above-described impurity. In use of a titanium dioxide, it is required in many cases to very strictly control the blended Ti content. Particularly, in the case of using the titanium dioxide as the dielectric raw material, the blended components must be controlled even to the ppm order. However, in industrial use, strict control of raw materials is not easy, because the water is a substance present in the atmosphere where the raw materials are handled, and great difficulties are involved in the control of the amount of chemically adsorbed water and physically adsorbed water on the particle surface.
The titanium oxide surface is fundamentally covered with an OH group chemically bonded to a Ti atom or an O atom, and a water molecule is physically adsorbed to this OH group in many layers by hydrogen bonding and forms a water content measured as the loss on drying (Manabu Kiyono, Sannka Titan (Titanium Oxide), p. 54, Gihodo Shuppan (1991)).
However, this water content is readily affected by the season or weather because moisture is repeatedly absorbed or released according to the ambient humidity. Therefore, in order to strictly control the ratio of BaO and TiO2, the materials must be bone-dry and weighed immediately before the synthesis and this imposes an large load in view of equipment and expense. Furthermore, as the particle is finer, the surface area per unit mass, namely, the specific surface area is larger and therefore, the amount of water adsorbed and the quantitative fluctuation of the raw material charged are larger. Combined with recent tendency toward fine particle formulation, the fluctuation of the charged amount and in turn, the reduction of the yield cannot be avoided.
The production process of titanium dioxide is roughly classified into a liquid phase process of hydrolyzing titanium tetrachloride or titanyl sulfate, and a vapor phase process of reacting titanium tetrachloride with an oxidative gas such as oxygen or water vapor. According to the liquid phase process, a titanium dioxide comprising anatase as the main phase can be obtained but this is in a sol or slurry state. In the case of using the titanium dioxide in this state, the application is limited. For using the titanium dioxide as a powder, the sol or slurry must be dried, but when dried, intensive aggregation generally results (Shinnroku Saito (superviser), Cho-Biryushi Handbook (Handbook of Ultrafine Particles), p. 388, Fuji-Technosystem Corporation, (1990)).
In the case of using this titanium dioxide as a photocatalyst or the like, the titanium oxide must be strongly cracked or ground so as to elevate the dispersibility, but this may cause problems such as mingling of abraded materials attributable to the grinding treatment or the like, and a non-uniform particle size distribution.
On the other hand, the titanium dioxide by a vapor phase process is excellent in the dispersibility as compared with that obtained by a liquid phase process, because a solvent is not used (Shinnroku Saito (superviser), Cho-Biryushi Handbook (Handbook of Ultrafine Particles), p. 388, Fuji-Technosystem Corporation, (1990)).
A large number of methods are known for obtaining ultrafine particulate titanium dioxide by a vapor phase process. For example, a process of producing a titanium dioxide by hydrolyzing titanium tetrachloride in flame is disclosed, wherein the reaction is performed by adjusting the molar ratio of oxygen, titanium tetrachloride and hydrogen to obtain a titanium dioxide having a high rutile content (JP-A-03-252315). Also, a process of producing a crystalline titanium dioxide powder by hydrolyzing titanium tetrachloride in a high-temperature vapor phase and rapidly cooling the reaction produce is disclosed, wherein the flame temperature and the titanium concentration in the raw material gas are specified to obtain a crystalline transparent titanium dioxide having an average primary particle diameter of 40 to 150 nm (JP-A-7-316536).
As for the process of producing a titanium dioxide comprising anatase as the main phase by a vapor phase process, for example, a production process where the rutile content ratio is adjusted by changing the ratio of hydrogen in an oxygen/hydrogen mixed gas in the vapor phase reaction is disclosed and a titanium dioxide having a rutile content of 9% is described, but the particle diameter of the titanium dioxide described is from 0.5 to 0.6 μm and coarser than the particle diameter range of particles generally called an ultrafine particle (JP-A-3-252315).
In the case of using a titanium dioxide for a photocatalyst or a solar cell, the fluctuation of loss on drying of titanium dioxide causes change in the formulation and this gives rise to fluctuation of quality and reduction of performance and yield.
Also, impurities such as Fe, Al, Si and S in the titanium dioxide give rise to a fluctuation in quality and reductions of performance and yield and therefor, their content is preferably reduced. For example, when Fe is present in titanium dioxide, coloration is caused and the titanium dioxide is not suited for usage where transparency is required. Also, when a component such as Al and S is present inside the titanium dioxide particle, crystal defects are generated and the function as a photocatalyst or a solar cell may be deteriorated.
As for the production process of titanium dioxide, when a titanium dioxide is produced by a vapor phase process starting from titanium tetrachloride, an ultrafine particle may be readily obtained, but chlorine originated in the raw material often remains in the titanium dioxide and dechlorination by heating, water washing or the like is required. The method for this treatment such as heating or water washing greatly affects the amount of water or hydroxyl group chemically adsorbing to the titanium dioxide particle surface. Such surface properties of titanium dioxide, including the residual chlorine, have a great effect not only on the amount of adsorbed water but also on the sintering or aggregation behavior of particles with each other at the heating in use of the titanium dioxide. Particularly, as the titanium dioxide particle is finer, the ratio of atoms present on the surface increases and the effect of the surface state becomes greater.
The present invention has been made to solve the above-described problems and an object of the present invention is to provide a fine particulate titanium dioxide with reduced fluctuation of the adsorbed water content which is a great mass fluctuation factor in a fine particulate powder body, more preferably a high-purity ultrafine particulate titanium dioxide and a production process thereof.